Alzheimer's disease (AD) is a progressive neurological disorder characterized by the loss of synaptic function eventually leading to neuronal death. One of the main histopathological features in AD is the extracellular deposition of insoluble amyloid aggregates of the amyloid-β (Aβ) peptide in neuritic plaques, but it is still unclear exactly how these deposits relate to the etiology of AD. The Aβ peptide is generally 39-43 amino acids long, usually 40 or 42, and is continuously generated throughout most of the body as the transmembrane amyloid-β (A4) precursor protein (APP) is proteolytically processed during normal cell functioning. The “amyloid cascade hypothesis” states that the imbalance in the production of the Aβ protein and its clearance from the brain is the initiating event, subsequently leading to neurodegeneration and dementia (Hardy, J. and Allsop, D. (1991) Trends Pharmacol. Sci. 12(10):383-388). There is indeed ample evidence that an increased Aβ burden brought about by genetic mutations dramatically increases the risk of developing AD (Gandy, S. (2005) J. Clin. Invest. 115:1121-1129). Although less than 3% of all AD cases carry such pathogenic mutations, it is very likely that the progressively increasing burden of Aβ is also the causative and toxic agent in the sporadic form (97% of all AD cases). Available diagnostic data also support that the initiating event in AD is Aβ deposition in the brain, which is detectable as a decrease in Aβ42 cerebral spinal fluid (CSF) levels (Jack, C. R. Jr. et al. (2010) Lancet Neurol. 9:119-128). This occurs well before the onset of symptoms, and pathology generally follows later. The increase in CSF “tau” (τ), on the other hand, is an indication of neurodegeneration—i.e. that nerve cells have already died, thus releasing the intracellular and hyperphosphorylated protein τ into the extracellular space. These elevated τ levels are only observed well after the onset of clinical symptoms. It thus follows that τ pathology probably succeeds Aβ pathology, and is most likely caused by it.
Although the Aβ peptide has been identified as the main culprit in AD, the mechanistic aspects of Aβ pathology remain elusive as Aβ has a high tendency to homo-oligomerize and is, in consequence, in a continuous equilibrium with higher-order aggregates. The result is a heterogeneous mix of monomeric, prefibrillar (both unstructured and with β-structure) and fibrillar oligomers of various sizes that continuously and irreversibly forms the insoluble fibrillar end state found in the plaques in AD patients' brains (a simplified aggregation scheme is depicted in FIG. 1). Recent data demonstrates that the oligomers of Aβ are particularly neurotoxic when compared to monomeric and fibrillar forms of the peptide and, therefore, that these oligomers are attractive therapeutic targets (Walsh, D. M. and Selkoe, D. J. (2007) J. Neurochem. 101:1172-84). The term “oligomer” is generally used to denote a molecule consisting of a plurality of monomers, and the term is herein reserved only for soluble species in order to distinguish them from the insoluble fibrils which are more polymeric than oligomeric. Herein, the terms “oligomer” and “soluble aggregate” are used interchangeably.
The oligomers formed by Aβ peptides during aggregation appear to be mainly of four types: (i) unstructured prefibrillar (of low-molecular weight, possibly micellar), (ii) prefibrillar with β-structure, (iii) prefibrillar of the “A11 positive” (A11+) type (of high-molecular weight, also containing β-structure), and (iv) fibrillar. These are schematically depicted in FIG. 1. Data on oligomers formed by the stabilized Aβ42-A21C/A30C mutant (Aβ42-CC), which renders the individual peptides incapable of adopting the fibrillar conformation, demonstrates that these prefibrillar oligomers with β-structure are 50 times more potent inducers of apoptosis than unstructured or fibrillar forms of the peptide (Sandberg, A. et al. (2010) Proc. Natl. Acad. Sci. USA 107:15595-15600). These oligomers are not of the A11+ type, and this result thus points to the prefibrillar oligomers with β-structure as the main mediators of Aβ toxicity. The A11+ aggregate has been suggested to be a special type of oligomer exposing a generic epitope found in several soluble aggregates of disease associated peptides and proteins (Kayed, R. et al. (2003) Science 300:486-489). The A11 epitope has also been found in native proteins as well, and it was thus suggested that it consists of exposed β-sheet edges (Yoshiike, Y. et al. (2008) PLoS ONE 3: e3235).
Although the exact etiological role of Aβ oligomers in AD is unknown, scientific evidence to date still points to the Aβ peptide as the most sensible target in AD therapy. However, many compounds and antibodies that have been effective against Aβ pathology in in vitro studies and in transgenic animal AD models have reached clinical trials, but none of them has proven effective in human Phase III trials. In consequence, current approved treatments for AD do not target Aβ specifically, but rather the underlying symptoms of the disease. These approved drugs are the acetylcholinesterase inhibitors and memantine (which is an N-methyl D-aspartate receptor antagonist).
Vaccination against the Aβ peptide was the first therapeutic strategy to exhibit efficacy in AD animal models. Naturally, clinical studies on AD patients ensued. But an early active vaccination trial on full-length partly aggregated Aβ42, termed AN-1792, was halted after an increased incidence of aseptic meningoencephalitis in immunized patients (Orgogozo, J-M. et al. (2003) Neurology 61:46-54). This adverse inflammation response was thought to be triggered by Th1 T-cell epitopes in the C-terminal part of the peptide. Ongoing active vaccination trials (ACC001; and CAD106) have therefore focused on using Aβ N-terminal fragments only. Ongoing passive immunization trials using monoclonal antibodies that have reached Phase II or later (Bapineuzumab, Solanezumab, and PF4360365) are advantageous over active strategies in that specific epitopes can be targeted. However, both Bapineuzumab and Solanezumab also target all Aβ indiscriminately by binding to generic epitopes in the N-terminal part of the peptide. The rationale has instead been that passive immunization allows for an increased control of antibody titers and, perhaps more importantly, that any adverse effects are quickly reversed by simply discontinuing the administration of antibody. PF4360365 is unique in that it targets the C-terminal of Aβ40 and not Aβ42 at all.
One important outcome of these vaccination trials is that plaque removal alone cannot prevent disease progression. This has been demonstrated in both Bapineuzumab and the AN-1792 trials (Holmes, C. et al. (2008) Lancet 372:216-223; Salloway, S. et al. (2009) Neurology 73:2061-2070); Rinne, J. O., et al. (2010) Lancet Neurol. 9:363-372). Furthermore, neither Bapineuzumab nor Solanezumab treatment has thus far been able to improve cognition in treated AD patients (Salloway, S. et al. (2009) Neurology 73:2061-2070; Siemers, E R. et al. (2010) Clin. Neuropharmacol. 33:67-73). Indeed, the strategy to remove all forms of Aβ by targeting generic epitopes has thus far proven to be ineffective. Without wishing to be bound by any theory, the present inventors believe that this may be a reflection of (i) the increasing awareness that monomeric or fibrillar Aβ aggregates may not be the toxic agent in AD and, therefore, that (ii) huge titers are required to also target the much less prevalent toxic oligomeric structures. But simply increasing dosage of antibody to also target the oligomers may prove problematic, as large doses of Bapineuzumab was shown to increase the risk of vascogenic oedema (swelling of the brain related to changes in blood vessels) thus necessitating a reduction in dosage (Salloway, S. et al. (2009) Neurology 73:2061-2070). It was speculated that this swelling was caused by a disproportionate targeting of Aβ aggregates on blood vessels (a condition known as cerebral amyloid angiopathy).
In addition to these reservations, there is also some uncertainty relating to the potential side-effects of targeting all Aβ indiscriminately, as it has recently been shown that Aβ peptides may play a pivotal role in regulating presynaptic function (Abramov, E. et al. (2009) Nature Neurosci. 12:1567-1576) and/or that it may normally function in the innate immune system (Soscia, S. J. et al. (2010) PloS ONE 5(3):e9505). It is here also worth noting that the Aβ peptide is produced during normal cell metabolism throughout most of the body, and not only in neuronal cells although the expression levels appear to be somewhat higher in both neuronal and glandular cells (the Human Protein Atlas at www.proteinatlas.org; Uhlén M. et al. (2005) Mol. Cell. Proteomics 4:1920-1932).
Since oligomers were identified as the primary therapeutic target in AD, attempts have been made to isolate antibodies specific for oligomers but not fibrils or monomers. Several of these, most notably antibodies of the IgM type, appear to exhibit stronger binding to oligomers based on an avidity effect alone (Lindhagen-Persson, M. et al. (2010) PLoS ONE 5:e13928). It is possible that this avidity effect is the primary reason for the higher apparent affinity for oligomers exhibited by many of the antibodies proposed to be “conformation specific”, even when these are of IgG type. Indeed, many of these antibodies also appear to bind fibrils, which is to be expected for such an effect. As an example: For the “conformation specific” antibody mAb158 one can still observe strong binding to fibrils (US 2009/0258009 A1). Other examples from prior art include antibodies 20C2 (US 2007/0218499 A1), hC2 (US 2010/0150906), 8F5hum8 (PCT/US 2008/065199), and NAB61 (US 2010/0209417 A1), which all exhibit high affinity for Aβ oligomers yet also display significant cross-reactivity with fibrils. Some of these “conformation specific” antibodies have now made it to human clinical trials. But it is at present still unclear if these antibodies exhibit a sufficiently high affinity for oligomers over monomers and/or fibrils in order for them not to be completely depleted by the preponderance of competing binding sites.